A similar instrument is known in accordance with the prior art portion of claim 1 (Czechoslovakian Pat. No. 133205).
However, the finger cuff used in that version of the instrument comprises a number of inflatable sacks in a rigid cylinder with light source and sensor mounted in the cylinder in such a way that the light must pass through the sacks as well as through the finger.
In accordance with the prior art, the idea of placing the photoelectric source-sensor pair directly against the skin, under the pressure cuff (made possible with the advent of miniature photoelectric sources and sensors), was put forward in 1975 by Reichenberger et al. (see Proceedings "Colloque International sur les Capteurs Biomedicaux", Paris, 1975, A7.5). Such a cuff was displayed in a demonstration of a valve and compressed air version of the instrument in Leiden, The Netherlands, in 1978 (Wesseling, K. H.: Niet invasieve vingerbloeddrukmeter, Boerhaave Lezingen, Wetenschappelijk rapport afd. Cardiologie, Academisch Ziekenhuis, Leiden, Oct. 6, 1978), and in Eindhoven, The Netherlands, in 1979 (Wesseling, K. H.: Bloeddrukmeting en een prototype vingerbloeddrukmeter, Colloquium Meten en Regelen, Technische Hogeschool Eindhoven, Afd. der Elektrotechniek en Technische Natuurkunde, June 8, 1979).
Such a cuff is virtually a miniaturized version of the conventional sphygmomanometer cuff, but incorporating an infrared light source and sensor. (Infrared light absorption is insensitive to blood oxygen changes or to changes in extravascular fluid volume resulting from the application of the cuff pressure to the finger.) Owing to the direct skin contact with the light source and sensor obtained in this type of cuff design, a much larger plethysmogram (the signal from the photoelectric sensor) can be obtained than with the original Czechoslovakian cuff design. Motion artifact is substantially reduced also as a result of this intimate contact with the finger plus the absence of the inertia (and thus motion relative to the finger) of a rigid cylinder. The cuff fits a large range of adult finger sizes (a smaller cuff is required for children). With this cuff design, the cuff air space is minimized, this being a major determinant of the size of the linear motor used in the present invention. The cuff length is determined by theoretical considerations of the longitudinal distribution of pressure transmitted from the cuff to the arterial wall, as well as, though to a lesser extent, the light source-sensor field pattern. A cuff length of minimally 4 cm allows accurate measurements to be obtained.
Moreover, in that earlier version (Czechoslovakian Pat. No. 133205) the electro-pneumatic transducer consists of an electrically controlled valve which controls the amount of compressed air shunted to the inflatable finger cuff or leaked off into the surrounding air. It has become apparent that this form of electro-pneumatic transducer represents a severely limiting factor in the operation of such an instrument, owing to the necessary presence of a flow constriction in the pneumatic circuit which thereby limits the speed at which the cuff can be inflated and thus the fastest component in the blood pressure that can be reliably tracked by the instrument. Such a restriction particularly degrades the performance of the instrument at higher heart rates and higher pulse pressures, besides severely limiting the maximum allowable length of flexible tubing connecting cuff and instrument thereby restricting the freedom of movement of the subject or patient. However, an even greater drawback of that earlier version is that its operation relies on the availability of a compressed air source. Whether that source be in metal bottle form or a bulky compressor motor, that version of the instrument cannot be considered to be self-contained.
Another instrument is known in accordance with the prior art employing the same underlying principle, but using a hydraulic system (see Yamakoshi et al., IEEE Transactions on Biomedical Engineering, vol. BME-27, no. 3, March 1980, pp. 150-153). However, such a hydraulic system precludes the use of a flexible finger cuff with all its inherent advantages, owing to the required rapid displacement of a significant mass of water from the prime mover to the cuff which would be required for this type of cuff in order to follow the blood pressure waveform. The resonance frequency of such a system is unacceptably low. Moreover, for this application, air is a much safer medium to work with than water, since leakages in the hydraulic system could be extremely hazardous for the patient.